There is great interest in the ability to make containers of plastic material for storing beverages, for example. This interest is due, among other factors, to the desire to reduce the weight of the packing used, as far as this is possible, and at the same time to utilize the good properties of plastic material, for example in respect of impact strength. The types of plastic material used for for this purpose are however, relatively expensive, so that in connection with the consumption of mateial it is obviously also necessary for reasons of economy to employ the smallest possible wall thickness of containers used. In order to withstand the loads to which the material packed in the containers is subjected, the parts of the containers which are subjected to these stresses, for example the bottom of a bottle, must be so constructed as to make the most effective possible use of the strength of the material. This is particularly important when containers of plastic material are used to store materials under pressure, for example, beverages containing carbon dioxide. Because of the requirements explained above, this means that the bottom of a container should have a mainly circular shape.
For a container, for example a bottle, for storing materials, it is an essential requirement that it should be possible for the container to be stored in an upright position. With a container having a bottom of circular shape this is obviously simply impossible. This has in turn the consequence that containers of the kind described here are equipped with supporting arrangements, for example feet. Various type of feet have been used, being either parts projecting from the bottom of the bottle or separate parts fixed to the containers. In the last-mentioned case these parts are applied to the container, for example, by snap engagement, riveting, or comparable methods.
The types of plastic which can be used at present within this field are mainly polyvinyl chloride (PVC), acetylonitrile (AN), and polyethylene terephthalate (PET), and materials similar to these. The reason for this is that the materials in question have viable combinations of barring properties and properties of impact resistance, and that they are economical and meet existing requirements for use as packages for food articles.
In order to acquire the desired properties in containers made of the above-mentioned materials, the material in the wall of the container is usually stretched so that it is oriented. This orientation will increase the impact resistance of the material. The containers are often made, in principle, as cylinders, where one end surface is more or less spherical, whereas the other end surface is often replaced by a bottle-like opening. The reason for the spherical end surface is that the materials mentioned have the property in common that they are relatively expensive, and that from a financial point of view the walls of the containers will therefore have to be made relatively thin. As the materials are elastic a shape of the end surface is required which will expose the material to as little strain as possible. The result of the spherical bottom which is therefore used is that a container made in this way cannot stand on a plane surface, e.g. on a table.
The advantages which are moreover a result of the use of these materials have brought about, however, that containers with spherical end surfaces have been equipped with supplementary devices so that the container has a standing surface. The solutions that have come forward can be divided into three groups:
According to the first group, the spherical end surface has been fitted with bulges of various shapes, so that the bulges form a kind of leg on the package.
In the second group the surface of the bottom, which is in principle spherical, has been dislocated inwards in its central part, so that the package can stand on the point between the extreme concave part of the spherical surface and the central inwards directed part of the bottom.
In the third group the containers are fitted with an outside stabilizing device which has the shape of a foot, so that the material in the outside stabilizing device is separated from the material in the container.
Containers which are made according to the characteristics of the first group mentioned above have the disadvantage that problems may arise in acquiring the necessary stability of deformation in the containers. Especially in connection with high temperatures the pressure in the containers can increase to such an extent that the legs are deformed to such a degree that the stability of the containers are reduced to an extent which is not desirable.
There is no doubt that the ability of the container to resist inside pressure can be increased by reducing the size of the legs, but this will only bring about the drawback that the standing surface of the container is so small already from the beginning that the stability of the standing container is much too small. Another drawback in connection with the containers discussed here is that they may be difficult to blow into the desired shape.
Containers made according to the second of the groups mentioned above will only have the required stability against inside pressure if the plastic material used is sufficiently stiff. Stiff plastic materials of the type mentioned above have the disadvantage, however, that they are brittle, and therefore such packages have reduced resistance to impacts, e.g. if they fall. The shape of the bottom mentioned will also entail that the standing surface is relatively small and that the stability of the containers is consequently lower than what is desired.
Finally, as far as the third group is concerned, it can be ascertained that it is usually provided with a foot made of an inexpensive material, in which the actual container is placed. The foot of the container is fastened to the container by means of some kind of snap connection. It may also occur that the foot is riveted mechanically to the package.
In the course of time, recycling of materials has gained more and more importance. This, no doubt, also is true for plastic materials. It is known that plastic materials may lose much of their good properties--the material may even become unfit for use--if it is contaminated by small quantities of another plastic material or contaminated by another material.
The usual procedure in connection with the manufacture of containers made of plastic is first to form a workpiece which is a tube that is closed at one end, which in the finished package is usually the bottom of the package. The workpiece is blown into the desired shape, e.g. that of a bottle. This method of manufacture means that there will be a thicker and less stretched part in the bottom of the finished package than in the other part of the package. This is usually the case in connection with the methods of manufacture used today. In certain cases, however, the mentioned central part can be prevented by using a flat, sheet-shaped workpiece for the manufacture of the container.